This invention relates to a control system for regulating the conduction angles of a network of SCR's (silicon controlled rectifiers) to vary the power translated from a three-phase A-C power source to a load circuit, while at the same time protecting the control system against undesired signal components in the three phase voltages that may otherwise preclude accurate control over the power delivered to the load circuit.
When the load circuit must be powered by an adjustable amplitude d-c voltage, such as is the case, for example, when the load takes the form of an inverter which in turn operates an a-c motor, the SCR network may comprise a three-phase full wave rectifier bridge having three pairs of SCR's, to each of which pairs is applied a respective one of the three alternating phase voltages provided by the three-phase A-C power supply and received from the power supply over three line conductors. The conduction angles of the six SCR's are controlled in order to establish at the output of the bridge a d-c voltage of a desired magnitude and to control the power supplied through the inverter to the a-c motor. More specifically, the power flow is adjusted by regulating the conduction angles of the SCR's during each half cycle of the applied a-c line voltage. Each SCR can conduct, during each half cycle of the voltage applied thereto from the three-phase A-C power source, when the SCR's anode is positive relative to its cathode. However, conduction will not occur during a half cycle until gate current is supplied to the SCR's gate. At that instant, the SCR fires into conduction, or turns on, and permits load current to flow therethrough until the end of the half cycle at which time the anode-cathode voltage will be zero and the anode will no longer be positive with respect to the cathode. "Conduction angle", as used herein and in the appended claims, means the time during which any SCR is actually conducting (the "on time") during a 180.degree. half-cycle of a supply voltage. The greater the angle or time delay between the start of a half cycle and the firing of the SCR into conduction, the less the conduction angle and the less alternating current that will be rectified and translated to the load circuit, namely, through the inverter to the a-c motor, thereby providing less d-c voltage across the output of the three-phase full wave rectifier bridge.
If controlled a-c power must be translated to a three-phase load, such as a three-phase a-c motor, a three-phase A-C switch may be utilized to vary the magnitude of the three alternating currents which flow to that load. Such a switch also includes three pairs of SCR's, each pair being connected in a respective one of the three line conductors over which the three phase voltages are received from the A-C power supply. By regulating the conduction angles of the six SCR's, the three alternating load or phase currents, supplied to the three-phase a-c motor, may be established at a desired level.
Regardless of the particular construction of the SCR network which adjusts the power flow, it is imperative that the gating signals for the SCR's be generated by circuitry which operates in precise synchronism with respect to the instants at which the phase voltages cross their a-c axis and thus have a zero instantaneous amplitude. The instants at which the SCR's are fired into conduction must be closely controlled and must be synchronized or keyed to the zero voltage points or crossings which will always occur at the same frequency and with the same time spacing or period from one zero crossing to the next, even though the phase voltages may not be perfectly sinusoidal in shape and may be contaminated with undesired harmonics, noise, transients or other distortion components. Unless the development of the gating signals for the SCR's is exactly synchronized to the zero voltage crossings of the three phase voltages, the timing of those gating signals will be incorrect and the SCR's will be triggered into conduction at the wrong times, resulting in an erroneous and undesired level of power flow to the load circuit. When the power flow cannot be properly regulated and held at a desired level, the reliability and performance of the entire system suffers substantially. Such misfiring of the SCR's can be caused by distortion components in the phase voltages. The gating may be too advanced or too retarded. If a relatively small conduction angle is required to achieve the desired power flow, the SCR's should be gated on near the end of each half cycle. A retarded gating signal could even allow gating near the start of the next half cycle, and this would effect much advanced gating for that next half cycle with a resultant surge in power flow which may damage or destroy some of the circuit components (such as the switching devices in an inverter) in the load.
Unfortunately, the previously developed SCR control systems for controlling the flow of three-phase A-C power are plagued by this problem. Such prior systems require relatively "clean" and sinusoidal shaped line voltages to operate properly. Distortion components on the power line deleteriously affect the operation of those prior systems to the extent that they cannot even be used in many power distribution systems where the three-phase line voltage is subject to distortion components.
This problem has been overcome by the present invention. The disclosed control system ensures that line voltage disturbances and distortions will not affect the power flow to the load circuit. The present invention provides a highly reliable control system which is immune to three-phase A-C power line distortions.
Previously-developed SCR control systems for three-phase power develop the gating signals for each phase by means of circuitry which is driven or operated only by that phase. In other words, individual and independently operated control circuits are employed for each of the three phase voltages, thereby requiring three complete and separate control circuits each having the same construction and each being controlled by a different one of the three phase voltages. In accordance with another aspect of the invention, a significant improvement over these prior systems is achieved in that a substantial portion of the control system is common to all three phases and yet is controlled by only one of the phase voltages. This results in a very efficient system, being considerably simpler and less expensive in construction and employing many fewer circuit components than the previous systems.